Method of reversibly staining a target cell

ABSTRACT

The present invention relates to methods of reversibly staining a target cell. The invention also relates to methods of isolating a target cell or a target cell population that is defined by the presence of at least one common specific receptor molecule. The invention also provides kits that can be used to carry out the methods of the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit of priority to U.S. provisional patent application 61/508,943 “Method of Reversibly Staining a Target Cell” filed with the US Patent and Trademark Office on 18 Jul. 2011, the contents of which is hereby incorporated by reference in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to methods of reversibly staining a target cell. The invention also relates to methods of isolating a target cell or a target cell population that is defined by the presence of at least one common specific receptor molecule. The invention also provides kits that can be used to carry out the methods of the invention.

BACKGROUND OF THE INVENTION

Cell therapy has proven to be highly effective for the treatment of a number of diseases. For example, primary immunodeficiencies can be cured by hematopoetic stem cell transplantation (HSCT) and some leukemia can be brought to complete remission by combined allogeneic HSCT and donor lymphocyte infusion (DLI) (Kolb, H. J. et al., Donor leukocyte transfusions for treatment of recurrent chronic myelogenous leukemia in marrow transplant patients. Blood 76 (12), 2462-2465 (1990). In some clinical settings, adoptive transfer of virus-specific T cells is very effective to reconstitute immunocompromised patients against life-threatening complications caused by cytomegalovirus (CMV) reactivation (Riddell, S. R. et al., Restoration of viral immunity in immunodeficient humans by the adoptive transfer of T cell clones. Science 257 (5067), 238-241 (1992)), or lymphoproliferative diseases mediated by Epstein-Barr-Virus (EBV) (Rooney, C. M. et al., Use of gene-modified virus-specific T lymphocytes to control Epstein-Barr-virus-related lymphoproliferation. Lancet 345 (8941), 9-13 (1995)). Similarly, tumor antigen-directed T cells, either derived from autologous tumor-infiltrating lymphocytes or cultured or engineered in vitro, are promising candidates for improved therapies.

Regardless of these interesting clinical observations, a broader transfer of cell therapy to clinical applications is still awaiting. This is mainly due to the fact that most procedures used to generate cell preparations for immunotherapy are very laborious, time consuming and expensive. Furthermore, cell populations known to mediate clinical effects usually need to be enriched to high purities, since contaminating ‘unwanted’ cells can mediate harmful and sometimes life-threatening side effects (like graft-versus-host-diseases (GvHD)-mediating alloreactive T cells upon allogeneic stem cell transplantation and/or DLI treatment). As it has been shown that already very low numbers of adoptively transferred T cells can contribute to beneficial clinical effects, similar rules will also apply for cell populations mediating negative side effects. Therefore, providing high purities of well-defined cell preparations applicable for therapy will become key to make these promising therapies more effective and predictable, as well as to lower the risk of potential side effects.

Current methods for surface marker-mediated clinical cell purification usually rely on single parameters (e.g. CD34, MHC multimers). However, for most cell populations—either directly derived ex vivo or after in vitro cell culture—a combination of different surface markers is necessary in order to truly segregate these cells from others. For example, naturally occurring regulatory T cells (Tregs) represent a promising cell subset, which might be able to prevent acute GvHD upon allogeneic HSCT 4 or the development of autoimmune diseases (Riley, J. L., June, C. H., & Blazar, B. R., Human T regulatory cell therapy: take a billion or so and call me in the morning. Immunity 30 (5), 656-665 (2009), Randolph, D. A. & Fathman, C. G., Cd4+ Cd25+ regulatory T cells and their therapeutic potential. Annu Rev Med 57, 381-402 (2006)).

Beside the expression of CD4, which is shared by a large number of cells, additional markers like their constitutive expression of the high-affinity IL-2 receptor a-chain (CD25) are needed to further narrow down heterogeneity. However, CD25 is also expressed on a large fraction of non-regulatory cells, which include recently activated effector and memory T cells. Therefore, combinatorial staining patterns, like combinations of CD4, CD25, CD127, and CD45RA (Miyara, M. et al., Functional delineation and differentiation dynamics of human CD4+ T cells expressing the FoxP3 transcription factor. Immunity 30 (6), 899-911 (2009), Hoffmann, P. et al., Only the CD45RA+ subpopulation of CD4+ CD25high T cells gives rise to homogeneous regulatory T-cell lines upon in vitro expansion. Blood 108 (13), 4260-4267 (2006)) have been suggested to more precisely identify this clinically relevant T cell subset.

All currently available clinical marker-based cell separation techniques utilize paramagnetic beads, which retain labeled cell populations within a magnetic field. Thereby, positive enrichment strategies using directly labeled target populations give highest purities. However, combinations of purifications via several different markers are still difficult to achieve, although being necessary to isolate distinct cell populations of highest purity. In addition, after positive selection, both labels and beads usually remain on the cell product, potentially manipulating the isolated cell population or negatively impacting its functionality/viability e.g. by receptor blockade. Especially in respect to clinical cell sorting, remaining cell labels cause substantial regulatory hurdles for the applicability of cell products into patients. In order to circumvent the problems of positive selection, many clinical cell processing procedures have been changed to depletion settings. Unfortunately, target cell purities are often poor and depletion methods often require a complicated cocktail of different antibodies, which makes their production and application laborsome and expensive.

Thus, there is still a need to provide a method for cell purification or isolation that allows, for example after cell sorting, the release and complete removal of all components of the receptor binding and staining reagents from the purified cell population.

SUMMARY OF INVENTION

In one aspect the invention provides a method of reversibly staining a target cell, said target cell comprising a receptor molecule on the surface thereof, with a detectable label, the method comprising:

contacting a mixture of cells comprising said target cell with (i) a receptor binding reagent, the receptor binding reagent comprising at least one (any) binding site B, wherein the binding site B specifically binds to said receptor molecule, wherein the dissociation rate constant (k_(off)) for the binding between said receptor binding reagent via the binding site B and said receptor molecule has a value of about 0.5×10⁻⁴ sec⁻¹ or greater, the receptor binding reagent further comprising at least one binding partner C, wherein the binding partner C is able of being (reversibly) bound to a binding site Z of a multimerization reagent, (ii) a multimerization reagent, the multimerization reagent comprising at least two binding sites Z for the reversible binding of the binding partner C of the receptor binding reagent, wherein the receptor binding reagent (i) and the multimerization reagent (ii) form (a plurality of) multivalent binding complexes that bind to said target cell, each multivalent binding complex comprising at least two of said receptor binding reagents bound to one said multimerization reagent, said multivalent binding complex providing increased avidity, relative to said receptor binding reagent; and (iii) said detectable label bound to said multivalent binding complex, wherein said target cell is stained by binding of said multivalent binding complex to said target cell, and wherein staining of said target cell is reversible upon disruption of the binding between said binding partner C of said receptor binding reagent and said binding sites Z of said multimerization reagent.

In a second aspect, the invention provides the use of such staining method for the isolation of a target cell population that is defined by the presence of a (at least one common specific) receptor molecule.

In a further aspect the invention provides a kit for reversibly staining a target cell (or isolating a target cell), said target cell comprising a receptor molecule on its surface, with a detectable label. Such a kit comprises:

(i) at least one receptor binding reagent comprising a (any) binding site B which specifically binds to said receptor molecule, wherein the dissociation rate constant (k_(off)) for the binding between said receptor binding reagent via the binding site B and said receptor binding molecule has a value of about 0.5×10⁻⁴ sec⁻¹ or greater, and the receptor binding reagent further comprising at least one binding partner C, wherein the binding partner C is able of being (reversibly) bound by a binding site Z of a multimerization agent, (ii) at least one multimerization reagent comprising at least two binding sites Z for said binding partner C of the receptor binding reagent, wherein the binding partner C and said binding sites Z of said multimerization reagent are capable of forming a reversible bond, (iii) a detectable label bound or capable of binding to the receptor binding reagent (i) and/or the multimerization reagent (ii).

In a further aspect, the invention provides the use of a receptor binding reagent which specifically binds to a receptor molecule, the receptor binding reagent comprising at least one binding site B, wherein the binding site B specifically binds to said receptor molecule, wherein the dissociation rate constant (k_(off)) for the binding between said receptor binding reagent via the binding site B and said receptor molecule has a value of about 0.5×10⁻⁴ sec⁻¹ or greater, for a method of reversibly staining a target cell.

In yet a further aspect, the invention provides a method of reversibly staining a target cell, said target cell comprising a (at least one) receptor molecule on the surface thereof, with a detectable label, the method comprising:

contacting a mixture of cells comprising said target cell with (i) at least two kinds of a receptor binding reagent, each kind of the receptor binding reagent comprising at least one binding site B, wherein the binding site B of each of the kind of receptor binding reagent specifically binds to said receptor molecule, wherein the dissociation rate constant (k_(off)) for the binding between each kind of said receptor binding reagent via the binding site B and said receptor molecule has a value of about 0.5×10⁻⁴ sec⁻¹ or greater, each kind of the receptor binding reagent further comprising at least one binding partner C, wherein the binding partner C of each kind of receptor binding reagent is able of being reversibly bound to a binding site Z of a multimerization reagent, (ii) a multimerization reagent, the multimerization reagent comprising at least one binding site Z for the reversibly binding of the binding partner C of each kind of the receptor binding reagent, wherein the at least two kinds of receptor binding reagent (i) and the multimerization reagent (ii) form (a plurality of) multivalent binding complexes that bind to said target cell, each multivalent binding complex comprising at least one of each kind of receptor binding reagent bound to one said multimerization reagent, said multivalent binding complex providing increased avidity, relative to each kind of said receptor binding reagent; and (iii) said detectable label bound to said multivalent binding complex, wherein said target cell is stained by binding of said multivalent binding complex to said target cell, and wherein staining of said target cell is reversible upon disruption of the binding between said binding partner C of each kind of said receptor binding reagent and said binding sites Z of said multimerization reagent.

In yet another aspect, the invention provides a reagent kit for reversibly staining (or isolating) a target cell, said target cell comprising a receptor molecule on the surface thereof, with a detectable label, the kit comprising:

(i) at least two kinds of a receptor binding reagent comprising a binding site B which specifically binds to said receptor molecule, wherein the dissociation rate constant (k_(off)) for the binding between each kind of said receptor binding reagent via the binding site B and said receptor binding molecule has a value of about 0.5×10⁻⁴ sec⁻¹ or greater, and each kind of the receptor binding reagent further comprising at least one binding partner C, wherein the binding partner C of each kind of the receptor binding reagent is able of being (reversibly) bound by a binding site Z of a multimerization reagent, (ii) at least one multimerization reagent comprising at least one binding site Z for said binding partner C of each kind of the receptor binding reagent, wherein the binding partner C of each kind of binding reagent and said binding sites Z of said multimerization reagent are capable of forming a reversible bond, (iii) a detectable label bound or capable of binding to the receptor binding reagent (i) and/or the multimerization reagent (ii).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

FIG. 1 depicts the principle of the reversible staining method of the invention using a Fab fragment as a (at least one) receptor binding reagent (in the case of a Fab fragment and also generally in one embodiment of the invention, the receptor binding reagent can have a single binding site for the receptor molecule). FIG. 1 a shows an illustrative example of a Fab fragment that is used as receptor binding reagent, in which a streptavidin binding peptide, the Strep-tag® is the binding partner C of the receptor binding reagent. In this embodiment the streptavidin binding peptide is fused or conjugated to the Fab fragment to form the receptor binding reagent. In this example as shown in FIG. 1 a, the multimerization reagent that comprises at least two binding sites Z for said at least one binding partner C is a streptavidin mutein (“Strep-Tactin®”). In the example of FIG. 1 a the multimerization reagent carries a fluorescent label such as phycoerythrin. In the example of FIG. 1 b again a Fab fragment is shown together with a multimerization reagent that is conjugated to a magnetic bead that serves as label. The reversible staining method using the multivalent binding complex formation between the streptavidin-mutein and a Fab fragment via the streptavidin-binding peptide is also herein referred to as “Fab-multimer staining”. FIG. 1 c depicts a schematic overview of this reversible Fab-multimer staining method. In this example, target cells are stained/isolated from a mixture of cells present in a blood sample. Fab-fragments with a k_(off) rate of about 0.5×10⁻⁴ sec⁻¹ or greater for the binding to a cell receptor molecule that is present on a target cell are reversibly multimerized by Strep-tag®/Strep-Tactin® complex formation. Target cells are stained with the multivalent binding complex (by contacting the cells with the multimerization reagent and the receptor binding reagent as shown in step a) of FIG. 1 c) and optionally separated from other cells that are devoid of the cognate cell surface receptor molecule, for example, by fluorescence-activated cell sorting (also known under the trademark “FACS™” in step b) of FIG. 1 c). Subsequent treatment of isolated stained target cells with the Strep-Tactin® ligand D-biotin competing with the Strep-tag® peptide causes displacement of the Fab fragments from the multimerization reagent, thereby releasing the multimerization reagent (Strep-Tactin® complex) from the target cell (as shown in step c) of FIG. 1 c). Remaining Fab fragments, uncomplexed from Strep-Tactin®, (spontaneously) dissociate in a reasonable time window due to their high k_(off) rate and can be completely removed from the target cell surface by washing (step d) of FIG. 1 c). The washing can be carried out in a volume diluting the Fab fragments during each washing step to at least to a concentration equaling to the affinity dissociation constant (K_(d)) for the binding between the Fab fragment and the cognate cell surface receptor molecule. FIG. 1 d shows the analogous staining procedure to that of FIG. 1 c in which the multimerization reagent is conjugated to a magnetic bead and in which the staining/separation step includes separation of stained cells from unstained cells via a magnetic column on which the stained cells are bound.

FIG. 2 shows the binding characteristics required for reversible Fab-multimer staining. FIG. 2 shows a FACS analysis of anti-CD4 Fab-multimer staining with different anti-CD4 Fab mutants with increasing k_(off)-rates for the binding of the Fab fragments to CD4 as a receptor molecule (a summary of the binding kinetics of the Fab fragments is listed in Table 1). Peripheral Blood Mononuclear Cells (PBMCs) were stained with anti-CD4 Fab fragments (receptor binding reagent) fused to a sequential arrangement of two Strep-tag® sequences (commercially available under the trade name “One-STrEP-tag”; binding partner C) and multimerized on phycoerythrin labeled Strep-Tactin® (Strep-Tactin PE; IBA GmbH, Göttingen, Germany; multimerization reagent comprising at least 2 binding sites Z for the binding partner C) and analyzed either before (second column) or after treatment with D-biotin (third column). Remaining Fab-monomers (i.e. Fab fragments used as receptor binding reagent) on the target cell surface were then detected after subsequent washing steps to remove the Fab monomers using Strep-Tactin PE alone (without bound Fab fragments) (fourth column). Fab-multimer staining after the reversible staining described above of the cells was performed again (fifth column) to control completion of biotin removal which otherwise would have hampered the staining of remaining Fab-monomers with Strep-Tactin PE alone (without bound Fab fragments) as shown in the fourth column and which thus would have led to a wrong conclusion of the experiment. Alternatively, cells were incubated with monomeric Fab-fragments, washed and subsequently analyzed after staining with Strep-Tactin® PE (first, i.e. left-most column). Live CD3⁺ T cells are shown. Numbers in the dot plots indicate the percentage of cells within gates.

FIG. 3 shows, alternatively to the FACS analysis of FIG. 2, a Western blot analysis of reversibility of Fab-multimer staining. CD4 Fab-multimers were generated with the different anti-CD4 Fab-mutants listed in Table 1 and further specified in FIG. 2 and Strep-Tactin® PE (IBA GmbH, Göttingen, Germany). PBMCs were incubated with the different CD4 Fab-multimers and consistent cell staining in each sample was verified by flow cytometry (solid line compared to the tinted histogram representing unstained cells). Then, cells were treated with D-biotin and subsequently washed. Finally, washed cells were lysed, and the soluble (S) and insoluble (I) fractions were analyzed for remaining Fab-monomers using highly specific anti-Strep-tag® antibodies (StrepMAB-Classic Horse Radish Peroxidase (HRP) conjugate; IBA GmbH, Göttingen, Germany). To control application of consistent amounts of insoluble cell material, β-actin was detected simultaneously with a primary antibody against β-actin from rabbit (Santa Cruz Biotechnology Inc., Santa Cruz, USA) and a secondary antibody against rabbit Ig (immunoglobulin) conjugated to HRP (Sigma, St. Louis, USA).

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for reversibly staining a target cell or a target cell population having a receptor molecule on the cell surface. The invention is based on the finding that, in contrast to the methods described in U.S. Pat. No. 7,776,562 or International Patent application WO02/054065, it is not essential for the reversibility of such a method that a low affinity binding is given between a receptor binding reagent and a receptor molecule on the surface of the target cell. Rather it has been found in the current invention that irrespective of the strength of the binding, meaning whether the dissociation constant (K_(d)) for the binding between the receptor binding reagent and the receptor molecule is of low affinity, for example, in the range of a K_(d) of about 10⁻³ to about 10⁻⁷ M, or of high affinity, for example, in the range of a K_(d) of about 10⁻⁷ to about 1×10⁻¹⁰ M, a target cell can be reversibly stained as long as the dissociation of the binding of the receptor binding reagent via the binding site B and the receptor molecule occurs sufficiently fast. Expressed in terms of the k_(off) rate (also called dissociation rate constant for the binding between the receptor binding reagent (via the binding site B) and the receptor molecule, the k_(off) rate is about 0.5×10⁻⁴ sec⁻¹ or greater, about 1×10⁻⁴ sec⁻¹ or greater, about 2×10⁻⁴ sec⁻¹ or greater, about 3×10⁻⁴ sec⁻¹ or greater, about 4×10⁻⁴ sec⁻¹ of greater, about 5×10⁻⁴ sec⁻¹ or greater, about 1×10⁻³ sec⁻¹ or greater, about 1.5×10⁻³ sec⁻¹ or greater, about 2×10⁻³ sec⁻¹ or greater, about 3×10⁻³ sec⁻¹ or greater, about 4×10⁻³ sec⁻¹, about 5×10⁻³ sec⁻¹ or greater, about 1×10⁻² sec or greater, or about 5×10⁻¹ sec⁻¹ or greater. The term “about” when used herein in relation to the k_(off) rate, the k_(on) rate or the K_(d) is meant to include an error margin of ±0.1%, ±0.2%, ±0.3%, ±0.4%, ±0.5%, ±0.7±0.9, %±1.0, %, ±1.2%, ±1.4%±1.6%, ±1.8,%±2.0%, ±2.2%, ±2.4,%, ±2.6%, ±2.8%, ±3.0%, ±3.5%, ±4.0. %, ±4.5%, ±5.0%, ±6.0%, ±7.0%±, 8.0%, ±9.0%±, 10.0%, ±15.0%, or ±20.0%.

In this context, it is noted that the formation of complex (C) between a receptor binding reagent L and its receptor P, for example, a cell surface receptor molecule can be described by a two-state process noted

C

P+L

The corresponding dissociation K_(d) constant is defined as

$K_{d} = \frac{\lbrack P\rbrack \lbrack L\rbrack}{\lbrack C\rbrack}$

wherein [P], [L], and [C] are the equilibrium molar concentrations of the receptor, the receptor binding reagent (ligand) and the respective complex at a given temperature and pressure. The dissociation K_(d) constant can also be expressed as the ratio of the constant of the on-rate (k_(on)) for the speed of association/formation (also called association rate constant) of the complex and the constant of the off-rate (k_(off)) for the dissociation of the complex (also called dissociation rate constant) with

K _(d) =k _(off) /k _(on)

In the present application, the values of the thermodynamic and kinetic constants K_(d), K_(a), k_(on) and k_(off) refer to their determination under “standard conditions”, i.e. a temperature of 25° C. and atmospheric pressure of 1.013 bar.

In the present invention, it has been found, as mentioned above, that the k_(off) rate [s⁻¹] for the binding of the receptor binding reagent to the receptor molecule via its specific binding site B is the determinant for the reversibility of staining of a target cell by a respective receptor binding reagent that has a further binding partner C, wherein the binding partner C is able of being bound by a multimerization reagent as defined herein. The k_(off) range can be chosen within a range, depending, for example, on the target cell to be stained and optionally to be purified and the respective experimental condition. Taking into account that the half-life T_(1/2) of the complex (C) between the receptor binding reagent L and the receptor molecule P can be expressed as ln2/k_(off), =0.693/k_(off) with a k_(off) of 0.5×10⁻⁴ sec⁻¹ it takes 13860 seconds or 231 minutes or 3.85 hours to reduce the concentration of the complex between the receptor binding reagent and the receptor molecule by half, assuming that the dilution is sufficient that rebinding of the dissociated receptor binding reagent to the receptor molecule can be neglected. For a k_(off) rate of 1.0×10⁻⁴ sec⁻¹, it takes thus 6390 seconds (or only 106.5 minutes or 1.775 hours), for a k_(off) of 2.0×10⁻⁴ sec⁻¹ it takes 3465 seconds (or 57 minutes. i.e. less than one 1 hour) and for a k_(off) of 4.0×10⁻⁴ sec⁻¹ it takes 1732 seconds (or about 28 minutes) to achieve the same reduction in the concentration of the complex. Thus, for the washing that is carried out after the separation of the stained target cells from the other cells in the sample and after the disruption of multivalent complex formed between the multimerization reagent and the receptor binding reagent, the cell receptor binding reagent may be chosen depending on the sensitivity of the cell to be stained to external influences. For example, for a rather sensitive cell, a receptor binding reagent with a rather high k_(off) rate of, for example, greater than 4.0×10⁻⁴ sec⁻¹ may be used so that, after the disruption of the multivalent binding complexes, most of the receptor binding reagent can be removed within one hour (as illustrated above, within 56 minutes the concentration of the complexes is reduced to 25% of the original concentration, assuming that rebinding effects can be neglected due to sufficient dilution). For a more robust cell, a receptor binding reagent with a lower k_(off) rate of, for example, 1.0×10⁻⁴ sec⁻¹, the dissociation of which needs accordingly much more time, may be used (in this case, 212 min or about 3 and a half hours are necessary to dissociate and remove 75% of the receptor molecule binding reagent). After dissociation of the multimerization reagent (for example, by means of an competitor that is able to disrupt the reversible bond between the binding partner C and the multimerization reagent), washing may also occur under constant gentle agitation for at least 2 times the half-life T_(1/2) in a buffer volume diluting the employed receptor binding reagent to a concentration which is at least by a factor of 10 below the dissociation constant (K_(d)). The dissociated receptor binding reagent is then removed, for example, by sedimentation of the target cells, and discarding the supernatant. In an alternative embodiment, it is possible to repeat the above described washing once or twice or several times to remove the receptor binding reagent to near completion. In this context, it is noted that the term “washing” as used herein with reference to the stained target cells means that the stained target cells are (concomitantly with the disruption of the multivalent binding complexes or after disruption of the multivalent binding complexes) contacted with a sufficient amount of washing buffer, incubated with the washing buffer over a suitable period of time for dissociation of the receptor binding reagent from the receptor molecule and then separated from the washing buffer (which thereby removes the dissociated receptor binding reagent) by a suitable procedure, for example, sedimentation of the target cells and removal of the supernatant or, for example, filtration of the washing buffer. The washing is usually carried out in buffer that does not have an adverse effect on the viability or the status of the cells to be stained or isolated, i.e. the washing is performed under physiologically acceptable conditions (such as physiologically acceptable buffer, suitable temperature, for example, at 4° C., 15° C. or 25° C., (i.e. room temperature) etc.). Suitable experimental washing conditions can easily be determined empirically by the person of average skill in the art. For example, if the k_(off) has a relatively high value close to 0.5×10⁻⁴ sec⁻¹, and the stained cells are (relatively) insensitive to higher temperature, the washing can be carried out at room temperature, to allow for a faster dissociation of the receptor binding reagent from the stained cells. Alternatively, if the stained cells are relatively sensitive to higher temperature, the washing might be carried out a 4° C., so that the dissociation/removal of the receptor binding reagent takes longer however the functional status or viability of the cells will not be affected. A temperature of 4° C. or at least below 15° C. is preferred in some embodiments to prevent any change of the physiological status of the target cells.

In this context, it is noted that the receptor binding reagent can either have a (any) single binding site B that specifically binds the receptor molecule or the receptor binding reagent can also have two or more such binding sites B, as long as binding of the receptor molecule via (each of) the binding site(s) B has an aggregate value of the k_(off) of about 0.5×10⁻⁴ sec⁻¹ or greater. Thus, the receptor binding reagent can be monovalent (for example, a monovalent antibody fragment or a monovalent artificial binding molecule (proteinaceous or other) such as a mutein based on a polypeptide of the lipocalin family (also known as “Anticalin®”), or a bivalent molecule such as an antibody or a fragment in which both binding sites are retained such as an F(ab′)₂ fragment. The receptor binding reagent might even be a pentameric IgM molecule, provided the k_(off) rate of the whole IgM molecule is 0.5×10⁻⁴ sec⁻¹ or greater.

The k_(off)-rate for the binding of the receptor binding reagent to the receptor molecule and, of course, also the k_(on) rate can be determined via standard methods, such as surface plasmon resonance (SPR), for example, using the BIAcore technology (SPR; Jonsson, U. et al. (1991) Biotechniques, 11, 620-627). This determination is within the average knowledge of the person skilled in the art. Usually, the determination is carried out at 25° C. by surface plasmon resonance analysis, at a suitable concentration of the receptor binding reagent) with the receptor molecule being immobilized on the surface of a respective sensor chip. The ligand (i.e., the receptor binding reagent), for example, a Fab fragment is applied on the chip in different concentrations (usually around the estimated K_(d) value as determined from preliminary characterizations), using flow rates in the range of μl/min.

As an illustrative example, it is referred in this context to the determination of the k_(off) rate of the binding of CD4 (as a receptor molecule) to Fab fragments derived from the monoclonal antibody 13B8.2. As described in U.S. Pat. No. 7,482,000 or in Bes, C., et al. J Biol Chem 278, 14265-14273 (2003), the k_(off) rate and the other kinetic parameters can be determined at 25° C. by surface plasmon resonance analysis using a BIAcore 2000 instrument (BIAcore AB, Uppsala, Sweden). Recombinant CD4 can be covalently immobilized on a CM5 sensor chip surface using the amine coupling method according to the manufacturer's instructions. A control reference surface can be prepared using the same chemical treatment of the flow cell surface without injection of CD4. Recombinant Fab mutants in buffer containing 10 mM Hepes (pH 7.4), 3 mM EDTA, 150 mm NaCl, and 0.005% nonionic surfactant P20 (BIAcore AB) can then injected at concentrations between 5 and 20 μg/ml over the flow cell, and the dissociation phase will follow by a regeneration step with 5 mM HCl solution. The flow rate can be set to 30 μl/min. All the sensorgrams can be corrected by subtracting the signal from the control reference surface. The data can then be globally fitted to a 1:1 Langmuir binding isotherm using BIAevaluation Version 3.2 software.

As another illustrative example it is referred to the determination of the k_(off) rate of anti-CD30 Fab fragments for the recombinant CD30A antigen fragment as measured by surface plasmon resonance by Schlapschy, M., et al. “Functional humanization of an anti-CD30 Fab fragment for the immunotherapy of Hodgkin's lymphoma using an in vitro evolution approach” Protein Eng Des Sel 17, 847-860 (2004). This determination was carried out on a BIAcore X system (BIAcore, Uppsala, Sweden). After buffer exchange to 10 mM sodium acetate pH 3.85 by gel filtration, purified CD30A were diluted to 50 μg/ml and immobilized on a ‘research grade’ CM5 sensor chip using the amine coupling kit (BIAcore), resulting in the immobilization of 1700 response units (RU). The purified recombinant Fab fragments were e applied in PBS/P (PBS containing 0.005% surfactant P-20; BIAcore) at a series of appropriate concentrations. Complex formation was observed under a continuous buffer flow of 5 μl/min. The sensorgrams were corrected by subtraction of the corresponding signals measured for the control blank channel, which was used in-line. The chip was regenerated by applying a 2 ml pulse of 1 mM NaOH at a flow-rate of 25 μl/min, followed by equilibration with PBS/P. Kinetic parameters can again be determined with BIAevalution software V 3.0 (BIAcore) using a 1:1 (Langmuir) binding model with drifting baseline.

Reverting to the method of the invention, as mentioned above, the affinity of the receptor binding reagent for the cell surface receptor molecule does not have to be of low affinity. Rather (see also the Experimental Section), the dissociation constant (K_(d)) for the binding between said receptor binding reagent and said receptor molecule can be the range of about 10⁻² M to about 10⁻⁸ M, or of about 10⁻² M to about 10⁻⁹ M, or of about 10⁻² M to about 0.8×10⁻⁹M, or of about 10⁻² M to about 0.6×10⁻⁹M, or of about 10⁻² M to about 0.4×10⁻⁹ M, or of about 10⁻² M to about 0.3×10⁻⁹ M, or of about 10⁻² M to about 0.2×10⁻⁹, or of about 10⁻² M to about 0.15×10⁻⁹M, or of about 10⁻² to about 10⁻¹⁰, as long as, after disruption of the multivalent binding complexes, the k_(off) allows the removal of the receptor binding reagent in a time frame and under washing conditions that are acceptable for the cell to be stained or isolated. In one embodiment the dissociation constant (K_(d)) for the binding between said receptor binding reagent and said receptor molecule can be in the range of about 10⁻⁷ M to about 10⁻¹⁰ M, or of about 10⁻⁷ M to about 0.8×10⁻⁹ M, or of about 10⁻⁷ M to about 0.6×10⁻⁹ M, of about 10⁻⁷ M to about 0.3×10⁻⁹ M, of 1.1×10⁻⁷ M to about 10⁻¹⁰ M, or of about 1.1×10⁻⁷ M to about 0.15×10⁻⁹M, or of about 1.1×10⁻⁷ M to about 0.3×10⁻⁹ M, or of about 1.1×10⁻⁷ M to about 0.6×10⁻⁹ M, or of about 1.1×10⁻⁷ M to about 0.8×10⁻⁹ M. In this context it is noted that k_(on)-rates up to about 5×10⁶ M⁻¹s⁻¹ are known for the binding of immunoglobulins such as antibodies or Fab-fragments to their respective antigen (see, for example, Lee et al., Mol. Biol. (2004) 340, 1073-1093, FIG. 4 in which the Fab fragment G6-23 has a k_(on) rate of 5.6×10⁶ M⁻¹s⁻¹ or, for example, Kwong et al J. Mol. Biol. 2008 Dec. 31; 384(5): 1143-1156 or International Patent Application WO 2010/017103 which reports k_(on) values of 1×10⁵ M⁻¹s⁻¹ to 1.4×10⁶ M⁻¹s⁻¹ for the Fab fragment of anti-human NKG2D monoclonal antibody and antibodies as IgG1 (see Table 1 of Kwong et al., supra). Accordingly, assuming a receptor binding reagent such as a Fab fragment has a k_(on) value for the binding of the receptor molecule of 5×10⁶ M⁻¹s⁻¹ and a K_(d) of 1×10⁻¹⁰ the k_(off) is 5×10⁻⁴ M, rendering such antibody molecules (or receptor binding reagents in general) well suitable for being used in the reversible staining method of the invention (with a k_(off) of 5×10⁻⁴ sec⁻¹ it takes only 1386 seconds or 23 minutes to reduce the concentration of the complex between the receptor binding reagent and the receptor molecule by half, despite an affinity with a K_(d) of 1×10⁻¹⁰ M). Likewise, as a further illustrative example, if the receptor binding reagent has a k_(on) value for the binding of the receptor molecule of 5×10⁵ M⁻¹s⁻¹ and a K_(d) of 1×10⁻⁹ the k_(off) is also 5×10⁻⁴ M, resulting also in a rather fast dissociation of the receptor binding reagent from the target cell, after disruption of the multivalent binding complexes. In this context, it is possible, for example, by using evolutionary methods such as phage display to deliberately screen for receptor binding reagents such as antibody fragments or, for example, lipocalin muteins (cf. also Lee et al., supra in this regard). Thus, if wanted, receptor binding reagents with desired kinetic properties can be obtained by such evolutionary methods from a starting molecule of which the amino acid sequence is known. It is also possible to use in the method described here a receptor binding reagent that has a low affinity for the receptor molecule, the dissociation constant (K_(d)) for the binding between the receptor binding reagent and the receptor molecule may then be in the range of 10⁻² to 10⁻⁷ M.

The staining method further comprises separating the stained cell from non-target cells present in a sample/mixture of cells. The separation can be carried out as described in U.S. Pat. No. 7,776,562 or International Patent application WO 02/054065 and can further comprise removing said staining from said cell by disrupting the binding between the binding partner C of the receptor binding reagent and the binding sites Z of the multimerization reagent. This bond should be reversible, i.e. the bond should be capable of being disrupted under conditions suitable for carrying out the claimed method. The dissociation constant (K_(d)) for the binding between said binding partner C and said binding sites Z can, for example, be in the range of about 10⁻² M to about 10⁻¹³ M. Thus, this reversible bond can, for example, have a K_(d) of between about 10⁻² M and about 10⁻¹³ M, or of between about 10⁻⁵ M and about 10⁻¹³ M, or of between about 10⁻⁶ M and about 10⁻¹⁰ M, or of between about 10⁻³ M and about 10⁻¹² M or between about 10⁻⁴ M and 10⁻¹¹ M, or between about 10⁻⁵ M and about 10⁻¹⁰ M. Also the K_(d) of this bond can be determined by any suitable means, for example, by fluorescence titration, equilibrium dialysis or surface plasmon resonance as explained above in connection with the determination of the k_(off) rate for the bond formed between the receptor binding reagent and the receptor molecule. The dissociation/removal of the staining of the multimerization reagent from the cells results in the removal of the dissociated receptor binding reagent, and thus of the whole multivalent binding complex including the detectable label from the previously stained cell. In order to facilitate the removal from the receptor binding reagent, a volume of a respective washing buffer might be used that brings the concentration of the added receptor binding reagent significantly below the K_(d) for its interaction with the cognate receptor molecule. Alternatively, the washing might be repeated several times at lower volumes and gentle agitation might be desirable to prevent immediate rebinding. In one embodiment a volume of the washing buffer is used that lowers the concentration of the receptor binding reagent below the K_(d) or even more preferably to a concentration below 1/10th of the K_(d) so that more than 90% of the receptor binding reagent is present in the dissociated form (i.e. not bound to the receptor molecule) or even more preferably to a concentration below 1/100th of the K_(d) so that more than 99% of the receptor binding reagent is present in the dissociated form or even more preferably to a concentration below 1/1000th of the K_(d) so that more than 99.9% of the receptor binding reagent is present in the dissociated form.

In some embodiments of the invention, the receptor binding reagent is selected such that it comprises at least one binding partner C and the multimerization reagent comprises at least two binding sites Z, at least three or at least four binding sites Z for the binding partner C. In alternative embodiments, it is possible to use two different (kinds of) receptor binding reagents. Each of these two receptor binding reagents bind the same receptor molecule by a same or a different binding site B (for the latter case, it is possible that the two receptor binding reagent recognize and thus bind different epitopes on the same receptor molecule). In addition, each of the two receptor binding reagents has at least one binding partner C and the multimerization reagent has at least one binding site Z for the binding partner C of each of the two different receptor binding reagents. For example, it is possible to use two different receptor binding reagents, which both bind the same receptor molecule via the same binding site B but which have different binding partners C1 and C2. For example, one receptor binding reagent may have a binding partner C1 such as a hexa-histidine tag and the other receptor binding reagent has a binding partner C2 such as a FLAG tag. In such case, the multimerization reagent can comprise only one binding site (Z1) for the binding partner C1 and only one binding site for the binding partner C2. Since in total two binding sites are present in the multimerization reagent an avidity effect is still achieved. Such a multimerization reagent might, for example, be a biocompatible polymer (for example, dextrane or another polysaccharide) to which an Fab fragment of an anti-FLAG antibody and one Fab fragment of an anti-hexahistidine tag antibody is conjugated (for a detailed discussion of multimerization reagents see below). Regardless, of whether one or two receptor binding reagents with only one or two or more binding partners C are used in the invention, any combination of the binding partner C and the multimerization reagent can be chosen, as long as the binding partner C and the binding site(s) Z of the multimerization reagent are able to reversibly multimerize in a (multivalent) complex to cause an avidity effect, as also described in U.S. Pat. No. 7,776,562 or International Patent application WO02/054065.

In some illustrative embodiments the partners can be chosen from the following group:

(a) the binding partner C comprises biotin and the multimerization reagent comprises a streptavidin analog or an avidin analog that reversibly binds to biotin, (b) the binding partner C comprises a biotin analog that reversibly binds to streptavidin or avidin and the multimerization reagent comprises streptavidin, avidin, a streptavidin analog, or an avidin analog that reversibly binds to said biotin analog, or (c) the binding partner C comprises a streptavidin or avidin binding peptide and the multimerization reagent comprises streptavidin, avidin, a streptavidin analog, or an avidin analog that reversibly binds to said streptavidin or avidin binding peptide. In some of these embodiments, the binding partner C may comprise the streptavidin-binding peptide Trp-Ser-His-Pro-Gln-Phe-Glu-Lys and said multimerization reagent comprises the streptavidin analog Val⁴⁴-Thr⁴⁵-Ala⁴⁶-Arg⁴⁷ or the streptavidin analog lle⁴⁴-Gly⁴⁵-Ala⁴⁶-Arg⁴⁷ both of which are described in U.S. Pat. No. 6,103,493, for example, and are commercially available under the trademark Strep-Tactin®. The streptavidin binding peptides might, for example, be single peptides such as the “Strep-tag®” described in U.S. Pat. No. 5,506,121, for example, or streptavidin binding peptides having a sequential arrangement of two or more individual binding modules as described in International Patent Publication WO 02/077018.

In other embodiments, in which only one binding partner C is used, the binding between the binding partner C and said at least 2 binding sites Z of said multimerization reagent occurs in the presence of a divalent cation. In an illustrative example of such an embodiment the binding partner C comprises a calmodulin binding peptide and the multimerization reagent comprises multimeric calmodulin as described in U.S. Pat. No. 5,985,658, for example. Alternatively, the binding partner C may comprise a FLAG peptide and said multimerization reagent may comprise an antibody binding to the FLAG peptide, e.g. the FLAG peptide, which binds to the monoclonal antibody 4E11 as described in U.S. Pat. No. 4,851,341. In yet another illustrative example the binding partner C comprises an oligohistidine tag and the multimerization reagent comprises an antibody or a transition metal ion binding the oligohistidine tag. The disruption of all these binding complexes may be accomplished by metal ion chelation, e.g. calcium chelation, e.g. by addition of EDTA or EGTA. Calmodulin, antibodies such as 4E11 or chelated metal ions or free chelators may be multimerized by conventional methods, e.g. by biotinylation and complexation with streptavidin or avidin or multimers thereof or by the introduction of carboxyl residues into a polysaccharide, e.g. dextran, essentially as described in Noguchi, A., Takahashi, T., Yamaguchi, T., Kitamura, K., Takakura, Y., Hashida, M. & Sezaki, H. (1992). “Preparation and properties of the immunoconjugate composed of anti-human colon cancer monoclonal antibody and mitomycin C dextran conjugate. Bioconjugate Chemistry 3, 132-137” in a first step and coupling of calmodulin or antibodies or chelated metal ions or free chelators via primary amino groups to the carboxyl groups in the polysaccharide, e.g. dextran, backbone using conventional carbodiimide chemistry in a second step.

In such embodiments the binding between said binding partner C and said at least two binding sites Z of said multimerization reagent can be disrupted by metal ion chelation. The metal ion chelation may, for example, be accomplished by addition of EDTA.

In the method of the invention it is also possible that the multimerization reagent is an oligomer or a polymer of streptavidin or avidin or of any analog of streptavidin or avidin. The oligomer or polymer may be crosslinked by a polysaccharide. In an embodiment oligomers or polymers of streptavidin or of avidin or of analogs of streptavidin or of avidin are prepared by the introduction of carboxyl residues into a polysaccharide, e.g. dextran, essentially as described in “Noguchi, A., Takahashi, T., Yamaguchi, T., Kitamura, K., Takakura, Y., Hashida, M. & Sezaki, H. (1992). Preparation and properties of the immunoconjugate composed of anti-human colon cancer monoclonal antibody and mitomycin C dextran conjugate. Bioconjugate Chemistry 3, 132-137” in a first step. Then streptavidin or avidin or analogs thereof are coupled via primary amino groups of internal lysine residue and/or the free N-terminus to the carboxyl groups in the dextran backbone using conventional carbodiimide chemistry in a second step. However, cross-linked oligomers or polymers of streptavidin or avidin or of any analog of streptavidin or avidin may also be obtained by crosslinking via bifunctional linkers such as glutardialdehyde or by other methods described in the literature.

In another embodiment of the staining method of the invention the binding partner C comprises an antigen and the multimerization reagent comprises an antibody or antibody fragment against said antigen. The antigen may, for example, be an epitope tag. Examples of suitable epitope tags include, but are not limited to, the Myc-tag (sequence EQKLISEEDL), the HA-tag (sequence: YPYDVPDYA), the VSV-G-tag (sequence: YTDIEMNRLGK), the HSV-tag (sequence: QPELAPEDPED), the V5-tag (sequence: GKPIPNPLLGLDST) or glutathione-5-transferase (GST), to name only a few. In another embodiment, the binding partner C comprises maltose binding protein (MBP), chitin binding protein (CBP) or thioredoxin as an antigen. In these cases, the complex formed between the at least two binding sites Z of the multimerization reagent (antibody) and the antigen can be disrupted by adding the free antigen, i.e. the free peptide such as a Myc-tag or the HA-tag (epitope tag) or the free protein (such as MBP or CBP). In this context, it is noted that in case the FLAG-tag (sequence: DYKDDDDK) is used as binding partner C and the multimerization reagent comprises an antibody or antibody fragment binding the FLAG tag, it is also possible of disrupting this reversible bond by addition of the free FLAG peptide.

In the method of the invention the at least one binding site B of the receptor binding reagent which specifically binds to said receptor molecule can for example be an antibody or a divalent antibody fragment such as an (Fab)₂′-fragment, divalent single-chain Fv fragment. It might also be a bivalent proteinaceous artificial binding molecule such as a dimeric lipocalin mutein that is also known as “duocalin”. In other embodiments the receptor binding reagent may have a single binding site B, i.e., may be monovalent. Examples of monovalent receptor binding reagents include, but are not limited to, a monovalent antibody fragment, a proteinaceous binding molecule with antibody-like binding properties or an MHC molecule.

Examples of monovalent antibody fragments include, but are not limited to an Fab fragment, an Fv fragment, and a single-chain Fv fragment (scFv).

Examples of proteinaceous binding molecules with antibody-like binding properties that can be used as receptor binding reagent that specifically binds the receptor molecule include, but are not limited to, an aptamer, a mutein based on a polypeptide of the lipocalin family, a glubody, a protein based on the ankyrin scaffold, a protein based on the crystalline scaffold, an adnectin, an avimer, a EGF-like domain, a Kringle-domain, a fibronectin type I domain, a fibronectin type II domain, a fibronectin type III domain, a PAN domain, a Gla domain, a SRCR domain, a Kunitz/Bovine pancreatic trypsin Inhibitor domain, tendamistat, a Kazal-type serine protease inhibitor domain, a Trefoil (P-type) domain, a von Willebrand factor type C domain, an Anaphylatoxin-like domain, a CUB domain, a thyroglobulin type I repeat, LDL-receptor class A domain, a Sushi domain, a Link domain, a Thrombospondin type I domain, an immunoglobulin domain or a an immunoglobulin-like domain (for example, domain antibodies or camel heavy chain antibodies), a C-type lectin domain, a MAM domain, a von Willebrand factor type A domain, a Somatomedin B domain, a WAP-type four disulfide core domain, a F5/8 type C domain, a Hemopexin domain, an SH2 domain, an SH3 domain, a Laminin-type EGF-like domain, a C2 domain, “Kappabodies” (Ill. et al. “Design and construction of a hybrid immunoglobulin domain with properties of both heavy and light chain variable regions” Protein Eng 10:949-57 (1997)), “Minibodies” (Martin et al. “The affinity-selection of a minibody polypeptide inhibitor of human interleukin-6” EMBO J 13:5303-9 (1994)), “Janusins” (Traunecker et al. “Bispecific single chain molecules (Janusins) target cytotoxic lymphocytes on HIV infected cells” EMBO J. 10:3655-3659 (1991) and Traunecker et al. “Janusin: new molecular design for bispecific reagents” Int J Cancer Suppl 7:51-52 (1992), a nanobody, a adnectin, a tetranectin, a microbody, an affilin, an affibody or an ankyrin, a crystallin, a knottin, ubiquitin, a zinc-finger protein, an autofluorescent protein, an ankyrin or ankyrin repeat protein or a leucine-rich repeat protein, an avimer (Silverman, Lu Q, Bakker A, To W, Duguay A, Alba B M, Smith R, Rivas A, Li P, Le H, Whitehorn E, Moore K W, Swimmer C, Perlroth V, Vogt M, Kolkman J, Stemmer W P 2005, Nat Biotech, December; 23(12):1556-61, E-Publication in Nat Biotech. 2005 Nov. 20 edition); as well as multivalent avimer proteins evolved by exon shuffling of a family of human receptor domains as also described in Silverman J, Lu Q, Bakker A, To W, Duguay A, Alba B M, Smith R, Rivas A, Li P, Le H, Whitehorn E, Moore K W, Swimmer C, Perlroth V, Vogt M, Kolkman J, Stemmer W P, Nat Biotech, December; 23(12):1556-61, E-Publication in Nat. Biotechnology. 2005 Nov. 20 edition).

In the method of the invention, the contacting of the mixture that contains the target cell(s) with the receptor binding reagent and the multimerization reagent can be carried out at any suitable temperature, as described in U.S. Pat. No. 7,776,562 or International Patent application WO 02/054065, for example. Typically, the contacting of the mixture containing the target cells with the receptor binding reagent and the multimerization reagent and later also the removal of said reagents may be carried out at such temperatures, at which substantially no activation and/or no signaling events occur, which might result in an alteration of the target cell, e.g. the T cell phenotype, in case a T cell is to be stained or isolated. In some more preferred embodiments the contacting is carried out at a temperature of ≦15° C. or carried out at a temperature of ≦4° C.

In the method of the invention, virtually any said target cell can be used that has at least one common receptor molecule that is used for staining or isolation of the target cell. In order to achieve an avidity effect, the receptor molecule is typically present in two or more copies on the surface of the target cell. In typical embodiments the target cell is a mammalian cell or a eukaryotic or prokaryotic cell. Likewise, the at least one common (specific) receptor which defines the target cell population may be any receptor against which a receptor binding reagent as described above may be directed. For example, the receptor may be an antigen defining a cell population or subpopulation, e.g. a population or subpopulation of blood cells, e.g. lymphocytes (e.g. T cells, T-helper cells, for example, CD4⁺ T-helper cells, B cells or natural killer cells), monocytes, or stem cells, e.g. CD34-positive peripheral stem cells or Nanog or Oct-4 expressing stem cells. Examples of T-cells include cells such as CMV-specific CD8+ T-lymphocytes, cytotoxic T-cells, memory T-cells and regulatory T-cells (Treg). An illustrative example of Treg are CD4⁺CD25⁺CD45RA Treg cells and an illustrative example of memory T-cells are CD62L⁺CD8⁺ specific central memory T-cells. The receptor may also be a marker for tumor cells. In this context, it is noted that the term “target cells” as used herein encompasses all biological entities/vesicles in which a membrane (which can also be a lipid bilayer) separates the interior from the outside environment and which comprise specific receptor molecules on the surface of the biological entity. Examples of such entities include, but are not limited to, viruses, liposomes and organelles such as mitochondria, chloroplasts, the cell nucleus or lysosomes.

Once being isolated, the target cell population is characterized in that the receptor binding reagent has been substantially completely removed (typically below the detection limit as, for example as being detected by FACS) from said receptor. The complete removal of the receptor binding reagent is accomplished by using the reversibly multimerized receptor binding reagent having a high k_(off) rate as described above. By so doing, a target cell population (defined by the common specific receptor) can be provided that has a functional status which has not been altered by the purification method.

The fact that the target cell population or any other population of a biological entity in which a membrane (which can also be a lipid bilayer) separates the interior from the outside environment and that is further characterized to comprise a common specific receptor molecule on the surface can be purified by the methods of the invention under subsequent removal of any used purification reagent (receptor binding reagent; multimerization reagent, label) offers—beyond the advantage that, if the target is a cell or an organelle, the physiological status is not altered—the regulatory advantage that the purification reagents are not administered to the patient during the use of such purified biological entities as medicaments. In such cases, regulatory authorities like FDA (USA) or EMEA (Europe) require less expensive constraints with respect to production processes for said purification reagents than in cases where the purification reagent is administered together with the medicament being a cell or a liposome. Therefore, a clear technical advantage exists also with respect to the methods of the invention for the purification of entities of which no physiological status can be manipulated like for liposomes, for example, if such liposomes have to be purified and are used as medicaments.

The label which is used for the detection of stained cells may be any label which is used in diagnostic and analytical methods. Preferably the label does not negatively affect the characteristics of the cells to be stained or isolated. Examples of labels are fluorescent labels (for example, phycoerythrin, allophycocyanin, coumarin or rhodamines to name only a few), magnetic labels, chromophoric labels, spin labels suitable for electron spin resonance/electron paramagnetic resonance (EPR), or radioactive labels. The label may be bound to the receptor binding reagent and/or the multimerization reagent. The label may be a direct label, i.e. a label bound to one of the members of the multivalent binding complex as specified above. In such a case, the label might, for example, be covalently coupled (conjugated) to either the receptor binding reagent or the multimerization reagent (cf. the multimerization reagent carrying a fluorescent label such as phycoerythrin shown in FIG. 1). Alternatively, the label may be an indirect label, i.e. a label which is bound to a further reagent which in turn is capable of binding to one of the members of the multivalent binding complex as specified above. Such a label may be added before, during or after the multivalent binding complex has been formed. An example for such an indirect label is a bis-, tris-, or tetrakis-NTA containing fluorescent dye as described by Lata et al., J. Am. Chem. Soc. 2005, 127, 10205-10215 or Huang et al., Bioconjugate Chem. 2006, 17, 1592-1600. Said label is able to bind non-covalently (via metal chelation) to an oligohistidine tag. Thus, in this example, the receptor binding reagent and/or the multimerization reagent may carry an oligohistidine tag (for example, a Fab fragment as receptor binding reagent that has an oligohistidine tag such as a hexa-histidine tag fused to the C-terminus of the CH1 or the CL-domain or a streptavidin mutein such as Strep-Tactin® as multimerization reagent that has an oligohistidine tag fused to the N- or C-terminus of one of its subunits can be used), thereby being enabled to non-covalently bind such a NTA based fluorescent dye compound described by Lata et al, supra or Huang et al, supra. Such a non-covalently binding label needs not necessarily be bound to its target (receptor binding reagent and/or the multimerization reagent) before the multivalent binding complex is formed but can also be added to the sample when the multivalent binding complex forms or after the multivalent binding complex has been formed. Such a non-covalently binding label can also be added after the multivalent binding complex is bound to the target cells. Instead of the NTA comprising fluorescent dye:oligohistidine tag binding pair described above, also any other specific binding pair such as, for example, digoxigenin and an anti digoxigenin antibody or antibody fragment carrying the fluorescent or other label can be used for indirect labeling. In such a case, the receptor binding reagent and/or the multimerization reagent is conjugated to/coupled with digoxigenin and an anti digoxigenin antibody or antibody fragment carrying the chosen label binds (via digoxigenin) to the multimerization reagent or the receptor binding reagent.

In embodiments of the method of the invention, a target cell is stained and optionally isolated by means of a group of at least two different (preselected) receptor molecules. Target cell isolation on behalf of more than one common receptor may be done by performing a method of the invention in a sequential manner by using corresponding receptor binding reagents in each cycle. In this case, the same label may be used for each cycle. Alternatively, simultaneous labeling of the target cells is possible when different labels are used for each receptor molecule. Such a sequential cell enrichment and/or (simultaneous) multiparameter staining uses at least two different receptor binding reagents, each receptor binding reagent comprising at least one binding site B which specifically binds to a preselected receptor molecule of a group of at least two different receptor molecules. Illustratively speaking, the at least two different receptor molecules may be different CD markers such as CD4⁺CD25⁺ T regulatory cells, or CD62L⁺CD8⁺ specific central memory T-cells. The cells can, for example, be antigen-specific T-cells such as CMV-specific CD8⁺ T cells which can be isolated by first selecting CD8⁺ T-cells, followed by a second staining and isolation for the CMV-specific target population such as HLA-B8/IE1K₁₉₉₋₂₀₇-specific CD8⁺ T-cells.

In such embodiments the receptor binding reagent that binds to the second or any further receptor molecule of the group of at least two different receptor molecules may further comprise at least one binding partner C.

For the staining of this second receptor molecule, the dissociation rate constant k_(off) for the binding between the receptor binding reagent that specifically binds via the binding site B and said second or any further receptor molecule may also have (like the receptor binding reagent that binds the first receptor molecule) a k_(off) rate of 0.5×10⁻⁴ sec⁻¹ or greater. Alternatively, in line with the method of U.S. Pat. No. 7,776,562 or International Patent application WO 02/054065, the dissociation constant (K_(d)) for the binding between said second or any further receptor molecule and said receptor binding reagent that specifically binds said second or any further receptor molecule may be in the range of 10⁻² to 10⁻⁷ M.

As discussed above the staining method of the invention can be used for the isolation of a cell population that is defined by the presence of at least one common specific receptor molecule. The cell population that is defined by the presence of a common specific receptor molecule may, for example, a T-cell population, including for example, an antigen-specific T cell population. This purified cell population can for example be used for adoptive transfer or immunotherapy.

The method of the invention can also be used for the determination of the functional status of a T cell population, or for the purification of a T cell population for in vitro expansion.

In more detail and in accordance with the disclosure of U.S. Pat. No. 7,776,562, the method of the present invention allows a functional isolation of target cell populations, e.g. of T cell populations based on a reversible staining procedure. The original functional status of target cells, e.g. T cells, can be substantially maintained after the identification and purification. Thus, the method of the invention is of broad benefit for basic research and clinical applications.

Examples of preferred applications are as follows:

Basic Research:

Direct ex vivo investigation of the functional status of lymphocytes such as T-cells including, for example, antigen specific T cell populations. The functional status of T cell populations in vivo is suggested to be highly diverse and dependent on specific in vivo conditions, but because of the lack of appropriate investigative tools, these aspects are only marginally understood. With, for example, Fab multimers as described here, epitope-specific T cell populations can be identified and purified independent of their functional status. However, the binding of irreversible reagents interferes with subsequent functional assays. Reversible T cell staining e.g. using Fab-fragments equipped with a streptavidin binding peptide/Strep-Tactin® reagents is a technology allowing the direct functional ex vivo investigation of unaltered diverse T cell populations.

Purification of T cell populations for highly efficient in vitro expansion. The characterization of T cell populations obtained ex vivo often requires further in vitro expansion to T cell lines or T cell clones.

With Fab multimer techniques, single cells or distinct phenotypic subpopulations within a diverse T cell population can be isolated, but the binding of the reagents to the TCR interferes with the efficiency of in vitro expansion. This experimental problem is solved by reversible T cell staining using the reagents of the invention, e.g. Fab-Strep-tag®/Strep-Tactin® reagents. Such Fab fragments might carry as binding partner C a streptavidin binding peptide having a sequential arrangement of two or more individual binding modules as described in International Patent Publication WO 02/077018.

Purification of T cell populations for adoptive transfer experiments. Many in vivo experiments in immunological research require the adoptive transfer of purified T cells into recipient animals. Both, the purity of the transferred T cell populations and possible changes in the cells that occur during the isolation procedure are concerns in these experimental systems. The highest purity of cell populations is achieved by positive selection methods, but the markers (usually antibodies) used for identification are difficult to remove from the surface of isolated cells and can interfere with the outcome of subsequent in vivo experiments.

Reversible T cell staining using reagents of the invention, e.g. Fab Strep-tag®/Strep-Tactin® reagents allows the combination of positive selection methods with later removal of the selection marker and might greatly improve adoptive transfer experiments.

Clinical Applications:

Purification of T cell populations for highly efficient in vitro expansion. Generation of human T cell lines or clones (e.g. pathogen/tumor-specific or autoreactive T cells) is necessary in many areas of clinical research, diagnostics, and immunotherapy. In vitro culture is often limited by difficulties in standardizing conditions for antigen-specific stimulation. Improved strategies for the purification of T cell populations could greatly enhance the efficiency of in vitro expansion, allowing the use of antigen-independent stimulation such as mitogens and anti-CD3. With the invention, e.g. with MHC-Strep-tag® or Fab-Strep-tag® reagents (as receptor binding reagent) and a Strep-Tactin® multimerization reagent, T cell populations can be isolated directly ex vivo and expanded in vitro after dissociation of the reagents. This approach is expected to be much more efficient than purification using, for example, conventional irreversible MHC multimer reagents, as the binding of the reagents negatively interferes with the efficiency of in vitro T cell expansion.

Purification of T cell populations from in vitro expanded cell lines or clones for further functional analyses or therapy. In vitro expansion of T cells requires the addition of antigen-presenting cells or feeder cells to the culture. For further functional analysis, and especially for therapeutic applications (e.g. adoptive transfer), it would be helpful to remove these contaminating cells. For positive selection procedures (which usually result in the highest degrees of purity), the selection marker should be removable from the T cell surface, as it might interfere with functional assays or adoptive transfer. If T cells are used for in vivo applications the selection marker must be further removed if it contains substances that could cause clinical complications such as allergic reactions. Reversible T cell staining e.g. using MHC-Strep-tag® or Fab-Strep-tag® reagents/Strep-Tactin® reagents fulfills all these criteria.

Ex vivo purification of T cell populations for “direct adoptive immunotherapy”. The isolation of T cell populations directly ex vivo followed by immediate transfer of the cells into recipients (without any further in vitro propagation) is of special clinical interest. It is expected that directly isolated cell populations are much more efficient than cultured cells for in vivo applications. Extremely high numbers of in vitro expanded T cells are required for effective adoptive transfers, a phenomenon most likely due to the adaptation of T cells to in vitro culture conditions. An example for an important clinical application for this procedure is the parallel purification and adoptive transfer of EBV and/or CMV-specific T cell populations during [otherwise] T cell-depleted stem cell transplantations, a protocol which is likely to dramatically reduce the incidence of EBV and CMV-related malignancies in transplant patients. Reversible T cell staining and isolation e.g. using MHC-Strep-tag® or Fab-Strep-tag® as receptor binding reagents and Strep-Tactin® as multimerization reagent is a very suitable method for these clinical applications. For example, MHC I molecules (fused to a streptavidin binding peptide) allow isolation of CMV-specific CD8+ T-cells in high purity from donor blood leukocytes; subsequently such purified cells are directly transferred to immunocompromised patients suffering from life threatening CMV disease (e.g. upon allogeneic stem cell transplantation). Cf. Schmitt et al., (2011) TRANSFUSION, Volume 51, pp. 591-599 in this respect. Other examples include purified Treg cell population for use in adoptive transfer for treatment of autoimmune diseases such as graft-versus-host disease (GvHD) or graft rejection, or Type 1 diabetes, colitis or allergy. For this purpose, the Treg target cell population may be CD4⁺CD25⁺CD45RA⁺ Treg cells which are stained/isolated in a sequential manner using three receptor binding reagents such as Fab fragments that specifically bind the CD4 receptor molecule, the CD25 receptor molecule or the CD45RA receptor molecule. Other examples include the isolation of natural killer cells via the CD56 receptor molecule (also known as Neural Cell Adhesion Molecule (NCAM)) or of hematopoietic stem cells such as CD34⁺, CD59⁺, Thy1/CD90⁺, CD38^(lo/−), CD133⁺ or Ckit/CD117⁺ hematopoietic stem cells via the respective cell surface receptor molecule.

Functional T Cell Diagnostics.

MHC multimer techniques allow quantification and phenotypic characterization of antigen-specific T cells directly ex vivo. However, binding of multimer reagents to the TCR complicates the use of purified cells in functional assays (e.g. chronic virus infections [HIV, CMV, EBV, HBV, HCV], tumor-specific T cell populations).

Reversible T cell staining and isolation e.g. using Fab-Strep-tag®/Strep-Tactin® reagents opens the door for powerful evaluation of such T cell status in many clinical situations.

The invention will be further illustrated by the following experimental Examples.

EXAMPLES Materials and Methods Blood Samples

Fresh PBMCs were generated from either peripheral blood or buffy-coats by centrifugation over Biocoll separating solution. Peripheral blood was obtained from healthy adult donors at the Institute of Medical Microbiology, Immunology and Hygiene (Technical University Munich), and buffy-coats were obtained from autologous blood donors at the Institute for Anesthesiology, German Heart Centre Munich (State of Bavaria and Technical University Munich). Written informed consent was obtained from the donors, and usage of the blood samples was approved according to national law by the local Institutional Review Board (Ethikkommission der Medizinischen Fakultait der Technischen Universitit Miinchen).

Production of Fab-Fragments as Receptor Binding Reagents (Clonin2, Expression, Purification)

The variable domains originating from the monoclonal anti CD4 antibody 13B8.2 were generated by gene synthesis using the sequences described in U.S. Pat. No. 7,482,000 and Bes, C., et al. J Biol Chem 278, 14265-14273 (2003). The obtained variable sequences were subsequently joined with sequences coding for the human constant region Ch1 type □1 for the heavy chain and kappa for the light chain, and the heavy chain was carboxy-terminally fused with a sequential arrangement of two streptavidin binding modules (SAWSHPQFEK(GGGS)₂GGSAWSHPQFEK), commercially available as One-STrEP-tag affinity tag from IBA GmbH, Göttingen, Germany. All cloning was done using the StarGate® cloning system (IBA GmbH, Göttingen, Germany) with fusion vectors adapted for the described Fab expression. Mutagenesis PCR was applied to introduce amino acid substitutions. Following cloning, the Fab-One-STrEP-tag fusion proteins were expressed in E. coli strain JM83 (Plückthun, A. & Skerra, A. Expression of functional antibody Fv and Fab fragments in Escherichia coli. Methods Enzymol 178, 497-515 (1989)). After periplasmatic expression, the Fab-fragments were purified by One-STrEP-tag/Strep-Tactin® affinity chromatography (Schmidt, T. G. & Skerra, A. The Strep-tag® system for one-step purification and high-affinity detection or capturing of proteins. Nat Protoc 2, 1528-1535 (2007)) via a Strep-Tactin® Superflow column (IBA), deliberated from desthiobiotin used for the elution of the respective Fab from the column by dialysis and thereby transferred into PBS pH 7.4 buffer and stored in PBS pH 7.4 at 4° C. or frozen (−20° C.) until use.

Multimerization, Staining and FACS Analysis

For staining of a CD4 receptor molecule specific target cell population in a mixture of cells, Strep-Tactin® labeled with phycoerythrine as a fluorescent label (IBA GmbH, Göttingen, Germany) was used as multimerization reagent together with the monomeric CD4 binding Fab-One-STrEP fragment of the variants of the antibody 13B8.2 (that served as receptor binding reagent) as described below. For FACS analysis 5×10⁶ PBMCs were incubated with Fab multimers consisting of 0.2 μg Fab-fragment (comprising binding site B) equipped with the One-STrEP-tag (binding partner C) as receptor binding reagent and 0.75 μg Strep-Tactin® PE (IBA GmbH, Göttingen, Germany) as multimerization reagent using the protocol below. In addition, after FACS separation, the multivalent complexes were disrupted in a portion of each of the stained cells by addition of biotin as described below. Control antibody stainings were performed by concomitant application of the respective antibody: anti-CD4 (OKT4) (from eBiosciences, San Diego, Calif., USA). After the staining procedure of the present invention, cells were subsequently stained with propidium iodide for live/dead discrimination.

Protocol for Staining of 5×10⁶ Cells with Receptor Binding Reagent and Multimerization Reagent 1. 0.75 μg Strep-Tactin PE were incubated with 0.2 μg monomeric CD4 binding Fab-One-STrEP fragment for 30 minutes at 4° C. in the dark. 2. 5×10⁶ pre-cooled cells were washed with 10 ml Buffer IS (0.5% BSA (w/v) in phosphate buffered saline (PBS) pH 7.4 with PBS=8.06 mM Na₂HPO₄ 1.47 mM KH₂PO₄, 137 mM NaCl) in a 15 ml reaction tube to remove potentially existing ingredients like, e.g., D-biotin, which interfere with the subsequent procedure. 3. The cells were resuspended in 50 μl Buffer IS and transferred to a reaction vessel suitable for staining, e.g. a 96-well round bottom microtiterplate. 4. The pre-incubated Fab-Streptamer preparation from step 1 was added to the cells and mixed thoroughly by gentle pipetting. 5. The mixture was incubated for 20 minutes at 4° C. in the dark. 6. The cells were washed three times by centrifugation (400×g, 2 min) in 200 μl Buffer IS. 7. Cells were now ready for FACS-analysis or FACS-sorting, FACS analysis was carried out on a CyAn ADP Lx (Beckman Coulter) and analyzed with FlowJo software (TreeStar). Protocol for Dissociation of Multivalent Complexes and Subsequent Removal of Receptor Binding Reagent from 5×10⁶ Cells with D-Biotin 1. 5×10⁶ stained cells (see above) were collected by centrifugation (400×g) and resuspended in 3 ml Buffer IS containing 1 mM D-biotin and incubated for 10 minutes at 4° C. 2. The cells were sedimented by centrifugation and the supernatant discarded. 3. Steps 1 and 2 were repeated. 4. The cells were resuspended in 10 ml Buffer IS and incubated for 10 minutes at 25° C. (for dissociation of the CD4 binding Fab fragment used as receptor binding reagent). 5. The cells were sedimented by centrifugation and the supernatant discarded. 6. Steps 4 and 5 were repeated three times.

Results Principle of Reversible Fab-Multimer Staining

The basis of the present invention was the surprising discovery that receptor binding reagents such as monomeric Fab molecules having high affinities for the cell receptor with a K_(d)<10⁻⁷ down to 10⁻¹⁰ M need to be multimerized by a multimerization reagent for stable binding to the target cells and can be completely removed from the cell surface upon targeted disruption of the interaction between partner C and binding sites Z of the multimerization reagent (FIG. 1). It was further discovered that the key molecular basis for such reversible staining is not the affinity constant K_(d) for the interaction between the cell surface receptor molecule and binding site B on the receptor binding reagent but the off rate for the dissociation of the receptor binding reagent from the receptor molecule. The off rate should be 0.5×10⁻⁴ sec⁻¹ or higher for that a receptor molecule binding reagent can be completely removed from the target cell within a reasonable time window.

In order to demonstrate the key finding that the off rate and not the affinity is the essential parameter for performing reversible Fab-multimer staining, the following recombinant Fab-fragments (serving as the receptor binding reagents) directed against the cell surface receptor molecule CD4 were generated:

-   -   1) An Fab fragment consisting of the variable domain of the CD4         binding murine antibody 13B8.2 and a constant domain consisting         of constant human CH1 domain of type gammal for the heavy chain         and the constant human light chain domain of type kappa, as         described in U.S. Pat. No. 7,482,000. This Fab is denoted in the         following 13B8.2 Fab fragment.     -   2) A mutant 13B8.2 Fab fragment in which the Phe residue at         position 100 of the heavy chain was mutated to Ala (referred to         herein also as CD4 mutant 1 (F100A) and referred in U.S. Pat.         No. 7,482,000, Table II, as F100K-H)     -   3) A mutant 13B8.2 Fab fragment in which the Tyr residue at         position 92 of the light chain was mutated to Ala (referred to         herein also as CD4 mutant 2 (Y92A) and referred in U.S. Pat. No.         7,482,000, Table III, as Y92-L)     -   4) A mutant 13B8.2 Fab fragment in which the H is residue at         position 35 of the heavy chain was mutated to Ala (referred to         herein also as CD4 mutant 3 (H35A) and referred in U.S. Pat. No.         7,482,000, Table II, as H35-H)     -   5) A mutant 13B8.2 Fab fragment in which the H is residue at         position 91 of the light chain was mutated to Ala (referred to         herein also as CD4 mutant 4 (H91A) and referred in U.S. Pat. No.         7,482,000, Table III, as H91-L)         These Fab fragments have a broad spectrum of affinities (K_(d)         values ranging from 12.5 nM to 16.9 μM) and increasing         k_(off)-rates; a summary of binding kinetics determined by         BIAcore is listed in the following Table 1.

TABLE 1 binding kinetics between immobilized CD4 and the Fab fragment of CD4 binding antibody 13B8.2 and mutants Fab fragments from U.S. Pat. No. 7,482,000 and Bes, C., et al.. J Biol Chem 278, 14265-14273 (2003) Fab fragment k_(on) (10⁴ s⁻¹ M¹ ⁾ k_(off) (10⁻⁴ s⁻¹) K_(d) (nM) CD4 wt  0.37 ± 0.09 ¹⁾  1.11 ± 0.4 ¹⁾  29.17 ± 3.27 ¹⁾ (13B8.2) CD4 mutant  0.562 ± 0.003²⁾  4.75 ± 0.05²⁾ 845.1 ± 47.5²⁾ 1 (F100A) CD4 mutant 7.27 ± 0.2³⁾ 10.80 ± 1.91³⁾ 14.75 ± 2.25³⁾ 2 (Y92A) CD4 mutant 0.0301 ± 0.200²⁾ 18.90 ± 0.85²⁾ 6279.1 ± 4240²⁾  3 (H35A) CD4 mutant 0.0239 ± 0.002²⁾ 40.40 ± 0.28²⁾ 16903.7 ± 1900²⁾  4 (H91A) ¹⁾ This value is the arithmetic mean of the 3 measurements given in Bes, et al, ²⁾Value of single measurement as given in Bes et al., ³⁾Value is the arithmetic mean of the 2 measurements given in Bes et al.,

In order to introduce a binding partner C, the heavy chains of CD4 binding Fab-fragments were genetically fused to a tandem arrangement of two Strep-tag®II sequences (SAWSHPQFEK(GGGS)₂GGSAWSHPQFEK commercially available as OneSTrEP-tag sequence from IBA GmbH, Göttingen, Germany), and both chains were simultaneously expressed in the periplasm of E. coli. The functionally assembled Fab-fragments that served as receptor binding reagent were then purified by affinity chromatography on a Strep-Tactin® resin and, after removal of desthiobiotin by dialysis, subsequently multimerized in the presence of water-soluble phycoerythrin-labeled Strep-Tactin® (Strep-Tactin® PE, which serves as multimerization reagent providing at least 2 (about 12) binding sites Z for the One-STrEP-tag sequence, with phycoerythrin serving as a (fluorescent) label). The interaction between the streptavidin binding peptide and the streptavidin mutein “Strep-Tactin®” is reversible upon addition of D-biotin (or its derivatives such as diaminobiotin or desthiobiotin, for example) as a competing ligand.

For staining and dissociation experiments, 5×10⁶ PBMCs were incubated with the respective anti-CD4 Fab-multimer-Strep-Tactin® PE complexes or with the corresponding Fab-monomers (the latter not complexed with Strep-Tactin® PE). Whereas the 13B8.2 wildtype (wt) Fab fragment and all mutants showed essentially equally good staining signals when multimerized, only the 13B8.2 wt Fab fragment and mutant 1, with slower k_(off)-rates (1.11×10⁻⁴ s⁻¹ and 4.75×10⁻⁴ s⁻¹), were able to bind to their antigen in a monomeric state (FIG. 2, first and second columns). After D-biotin-mediated disruption of the multimeric complexes (FIG. 2, third column) and subsequent washing, the cells were probed for residual surface-bound Fab fragments by the addition of Strep-Tactin® PE alone (without bound Fab) (FIG. 2, fourth column). As expected from the monomer staining experiments, no remaining Fab monomers could be detected for mutants 2, 3 and 4, whereas substantial residual cell surface bound Fab of the wild-type and mutant 1 variants was observed. For mutants 2, 3 and 4, cells could be efficiently re-stained a second time using Fab-multimer labeling once again (FIG. 2, fifth column) demonstrating consistent biotin removal during detection of residual Fab fragments on the cell surface.

In addition to flow cytometry-based analysis (FIG. 2), complete removal of Fab-multimers was also assessed by an independent other analysis method, i.e. Western blot analysis (FIG. 3). Confirming the highly sensitive FACS results shown in FIG. 2, no Fab-fragments could be detected in either the soluble or the insoluble cell fraction after cell lysis for mutants 2, 3, and 4 while the wild type Fab fragment could be readily detected in the soluble fraction. As this experiment would also have detected internalized Fab fragments, which would not have been detected by FACS, any significant internalization of surface-bound receptor binding reagents during the staining and release procedure could be ruled out, thereby confirming correct interpretation of the results shown in FIG. 2. In contrast to the FACS experiment demonstrating incomplete removal of mutant 1 under the used washing conditions, remaining mutant 1 could not be detected in similarly treated cells by the Western blot experiment, presumably due to the inferior sensitivity of the Western blot method.

The main surprising result is the following: While, due to their low affinity (K_(d) of about 6.2 μM and 16 μM) the complete release for Fab mutants 3 and 4 after disruption of the multivalent complex of the respective Fab fragment and Strep-Tactin® PE as the multimerization reagent was in line with the teaching of U.S. Pat. No. 7,776,562 or International Patent application WO02/054065, the complete release of the mutant 2 Fab-fragment in contrast to monomeric wild-type Fab molecules (and similar to the mutants 3 and 4 Fab fragments) was completely unexpected, since the affinity (K_(d)=14.75±2.25 nM) of mutant 2 was determined to be significantly higher than for the wild-type (K_(d)=29.17±3.27 nM) and than for all other mutants. However, mutant 2 is characterized by a much faster off-rate (k_(off)=10.80±1.91×10⁻⁴ s⁻¹; see also Table 1) than the wild-type Fab fragment and Fab mutant 1, showing that the rate of dissociation has a dominating effect on the binding stability of surface-bound, monomeric Fab-fragments and not the affinity constant. In line with this interpretation, all fully reversible Fab-fragments (mutants 2-4) (which are the receptor binding reagents as defined herein) share a fast k_(off)-rate that is greater than 0.5×10⁻⁴ s⁻¹. However, it should also be noted here that, during disruption of the multivalent binding complexes, with longer washing/incubation with the D-biotin containing solution, complete reversal of the binding of the wild type Fab fragment and Fab mutant 1—which both also have a k_(off)-rate that is greater than 0.5×10⁻⁴ s⁻¹—is possible within a reasonable time window and thus can be used as receptor binding reagents in accordance with the invention.

In summary, these data demonstrate that multimerized receptor binding reagents with a k_(off)-rate for the binding of a receptor molecule via the binding site B that is greater than 0.5×10⁻⁴ sec⁻¹ (for example Fab-fragments) can be used to stain cells in a stable manner and that these reagents can be completely detached and removed from the surface of labeled cells under physiological conditions.

The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising”, “including,” containing”, etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by exemplary embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein

Other embodiments are within the following claims. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group. 

1. A method of reversibly staining a target cell, said target cell comprising a receptor molecule on the surface thereof, with a detectable label, the method comprising: contacting a mixture of cells comprising said target cell with (i) a receptor binding reagent, the receptor binding reagent comprising at least one binding site B, wherein the binding site B specifically binds to said receptor molecule, wherein the dissociation rate constant (k_(off)) for the binding between said receptor binding reagent via the binding site B and said receptor molecule has a value of about 0.5×10⁻⁴ sec⁻¹ or greater, the receptor binding reagent further comprising at least one binding partner C, wherein the binding partner C is able of being reversibly bound to a binding site Z of a multimerization reagent, (ii) a multimerization reagent, the multimerization reagent comprising at least two binding sites Z for the reversible binding of the binding partner C of the receptor binding reagent, wherein the receptor binding reagent (i) and the multimerization reagent (ii) form multivalent binding complexes that bind to said target cell, each multivalent binding complex comprising at least two of said receptor binding reagents bound to one said multimerization reagent, said multivalent binding complex providing increased avidity, relative to said receptor binding reagent; and (iii) said detectable label bound or capable of binding to said multivalent binding complex, wherein said target cell is stained by binding of said multivalent binding complex to said target cell, and wherein staining of said target cell is reversible upon disruption of the binding between said binding partner C of said receptor binding reagent and said binding sites Z of said multimerization reagent.
 2. The method of claim 1, wherein the dissociation constant (K_(d)) for the reversible binding between said binding site Z and said partner C is in the range of 10⁻² M to 10⁻¹³ M.
 3. The method of claim 1, wherein the dissociation rate constant (k_(off)) for the binding between said receptor molecule binding reagent and said receptor molecule has a value of about 1×10⁻⁴ sec⁻¹ or greater, of about 2×10⁻⁴ sec⁻¹ or greater, or about 3×10⁻⁴ sec⁻¹ or greater, of about 4×10⁻⁴ sec⁻¹ or greater, of about 5×10⁻⁴ sec⁻¹ or greater, of about 1×10⁻³ sec⁻¹ or greater, of about 1.5×10⁻³ sec⁻¹ or greater, of about 2×10⁻³ sec⁻¹ or greater, of about 3×10⁻³ sec⁻¹ or greater, of about 4×10⁻³ sec⁻¹, of about 5×10⁻³ sec⁻¹ or greater, or about 1×10⁻² sec⁻¹ or greater.
 4. The method of claim 1, wherein the dissociation constant (K_(d)) for the binding between said receptor binding reagent and said receptor molecule is in the range of 10⁻² to 10⁻¹⁰ M.
 5. The method of claim 4, wherein the dissociation constant (K_(d)) for the binding between said receptor binding reagent and said receptor molecule is in the range of about 10⁻³ M to about 10⁻¹⁰ M, or about 10⁻³ M to about 10⁻⁹ M, or about 10⁻³ M to about 10⁻⁸ M, or about 10⁻³ M to about 10⁻⁷ M, or about 10⁻⁷ M to about 10⁻¹⁰ M, or about 10⁻⁷ M to about 10⁻⁹ M.
 6. (canceled)
 7. The method of claim 1 further comprising removing said staining from said target cell by disrupting the binding between said binding partner C of the receptor binding reagent and said binding sites Z of said multimerization reagent.
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 9. The method of claim 1, wherein (a) said binding partner C comprises biotin and said multimerization reagent comprises a streptavidin analog or an avidin analog that reversibly binds to biotin, (b) said binding partner C comprises a biotin analog that reversibly binds to streptavidin or avidin and said multimerisation reagent comprises streptavidin, or avidin, or a streptavidin analog, or an avidin analog that reversibly binds to said biotin analog, or (c) said binding partner C comprises a streptavidin or avidin binding peptide and said multimerization reagent comprises streptavidin, or avidin, or a streptavidin analog, or an avidin analog that reversibly binds to said streptavidin or avidin binding peptide.
 10. The method of claim 9, wherein said binding partner C comprises the streptavidin-binding peptide Trp-Ser-His-Pro-Gln-Phe-Glu-Lys and said multimerization reagent comprises the streptavidin analog Val⁴⁴-Thr⁴⁵-Ala⁴⁶-Arg⁴⁷ or the streptavidin analog lle⁴⁴-Gly⁴⁵-Ala⁴⁶-Arg⁴⁷.
 11. The method of claim 1, wherein the binding between said binding partner C and said binding sites Z of said multimerization reagent occurs in the presence of a divalent cation.
 12. The method of claim 11, wherein said binding partner C comprises a calmodulin binding peptide and the said multimerization reagent comprises calmodulin, or wherein said binding partner C comprises a FLAG peptide and said multimerization reagent comprises an antibody binding the FLAG peptide, or wherein said first binding C comprises an oligohistidine tag and said multimerization reagent comprises an antibody binding the oligohistidine tag.
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 15. The method of claim 1, wherein said multimerization reagent is an oligomer or a polymer of streptavidin or avidin or of any analog of streptavidin or avidin.
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 17. The method of claim 1, wherein said binding partner C comprises an antigen and said multimerization reagent comprises an antibody against said antigen.
 18. The method of claim 17, wherein the antigen is an epitope tag or wherein said antigen comprises a protein.
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 22. The method of claim 1, wherein said receptor binding reagent which specifically binds to said receptor molecule is selected from the group consisting of an antibody, an divalent antibody fragment, a monovalent antibody fragment, a proteinaceous binding molecule with antibody-like binding properties and an MHC molecule.
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 28. The method of claim 1, wherein said target cell is a mammalian cell.
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 33. The method of claim 1, wherein a target cell is subsequently stained and isolated by means of a group of least two different receptor molecules, using at least two different receptor binding reagents, each receptor binding reagent comprising a binding site B which specifically binds to the preselected receptor molecule of the group of the at least two different receptor molecules,
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 37. The method of claim 1 comprising isolating cell population that is defined by the presence of at least one common specific receptor.
 38. The method of claim 37 wherein said cell population that is defined by the presence of a common specific receptor is a lymphocyte population or a stem cell population.
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 53. A reagent kit for reversibly staining a target cell, said target cell comprising a receptor molecule on the surface thereof, with a detectable label, the kit comprising: (i) at least one receptor binding reagent comprising a binding site B which specifically binds to said receptor molecule, wherein the dissociation rate constant (k_(off)) for the binding between said receptor binding reagent via the binding site B and said receptor binding molecule has a value of about 0.5×10⁻⁴ sec⁻¹ or greater, and the receptor binding reagent further comprising at least one binding partner C, wherein the binding partner C is able of being reversibly bound by a binding site Z of a multimerization agent, (ii) at least one multimerization reagent comprising at least two binding sites Z for said binding partner C of the receptor binding reagent, wherein the binding partner C and said binding sites Z of said multimerization reagent are capable of forming a reversible bond, (iii) a detectable label bound or capable of binding to the receptor binding reagent (i) and/or the multimerization reagent (ii).
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 60. A method of reversibly staining a target cell, said target cell comprising a receptor molecule on the surface thereof, with a detectable label, the method comprising: contacting a mixture of cells comprising said target cell with (i) at least two kinds of a receptor binding reagent, each kind of the receptor binding reagent comprising at least one binding site B, wherein the binding site B of each of the kind of receptor binding reagent specifically binds to said receptor molecule, wherein the dissociation rate constant (k_(off)) for the binding between each kind of said receptor binding reagent via the binding site B and said receptor molecule has a value of about 0.5×10⁻⁴ sec⁻¹ or greater, each kind of the receptor binding reagent further comprising at least one binding partner C, wherein the binding partner C of each kind of receptor binding reagent is able of being reversibly bound to a binding site Z of a multimerization reagent, (ii) a multimerization reagent, the multimerization reagent comprising at least one binding site Z for the reversibly binding of the binding partner C of each kind of the receptor binding reagent, wherein the at least two kinds of receptor binding reagent (i) and the multimerization reagent (ii) form multivalent binding complexes that bind to said target cell, each multivalent binding complex comprising at least one of each kind of receptor binding reagent bound to one said multimerization reagent, said multivalent binding complex providing increased avidity, relative to each kind of said receptor binding reagent; and (iii) said detectable label bound or capable of binding to said multivalent binding complex, wherein said target cell is stained by binding of said multivalent binding complex to said target cell, and wherein staining of said target cell is reversible upon disruption of the binding between said binding partner C of each kind of said receptor binding reagent and said binding sites Z of said multimerization reagent.
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